Information Technology Reference
In-Depth Information
one FAT is considered valid, the Flags specify which one. A FAT16 BPB
has no Flags field.
• Some fields that have identical functions are stored in different locations
in FAT16 and FAT32 BPBs.
• The boot code, if present, begins at byte 90.
For maximum compatibility with mass-storage host software, several items
in a FAT32 boot sector have recommended values, as shown in Table 8-4. In
theory all of the values shown can vary, but straying from the recommended
values can cause problems with some hosts.
As in a FAT16 BPB, byte 13 stores the number of sectors per cluster.
A FAT32 boot sector can contain boot code in bytes 90-509 and the sector
must have a boot signature of AA55h in bytes 510-511. The jump instruc-
tion at offset 00h is typically EBh 58h 90h, which means jump ahead 88
bytes to the beginning of the boot code at 5Ah.
The FSInfo Structure
The FSInfo structure can contain information to help the master computer
find free clusters quickly. The structure is in the location specified in byte 48
in the BPB. The location is typically in reserved sector 1, immediately fol-
lowing the boot sector. Table 8-5 shows the contents of the FSInfo structure.
The Backup Boot Sector
Sectors 6-8 in the reserved region can store a backup copy of the three sec-
tors beginning with the volume's boot sector. The backup is for use in media
repair if the original copy is damaged.
File Allocation Table Region
As with FAT16, following the reserved region are two copies of the file allo-
cation tables (FATs). A FAT32 table has a 32-bit entry for each data cluster
in the volume. The highest four bits are reserved, however. During format-
ting, all 32 bits of each entry are set to zero. After formatting, software that
reads and writes to the FAT should preserve the contents of the four high
bits.
The Flags field in the BPB specifies whether or not the FATs are mirrored. If
mirroring is enabled, the file-system driver maintains two identical copies of
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